1. Field of the Invention
The present invention relates to digitization of signals read from a recording medium, and more particularly, to data slicing circuitry and data slicing method used in digitizing a signal read from a recording medium, such as an optical disk.
2. Description of Related Art
As a storage medium for storing information such as data, image and sound, optical disks, such as compact disks (CDs), magneto-optical disks (MOs) and digital video disks (DVDs), have been developed and their capacities are increasing.
Since data bits are stored at a high density on an optical disk having a large recording capacity, a signal read from the optical disk tends to be affected by so-called "inter-symbol interference." The inter-symbol interference, also called "inter-wave interference," is such a phenomenon that read signals interfere with signals read for adjacent data and the read signals are changed from the ideal waveforms.
The effect of inter-wave interference is particularly distinctive, for example, when data is a combination of "3T" (shortest data)-"8T" (longest data) patterns in "2-7" code, for example. That is, as is shown in FIG. 1, if long data such as "8T" are present and adjacent read waveform for the "3T" pattern is reduced from waveform 10 to waveform 11 by inter-wave interference. In other words, the effect of inter-wave interference varies depending on how the data patterns are combined.
As a result, when the signal is digitized by a so-called "PWM" system for digitizing signals based on pulse width, as is shown in FIG. 1 the pulse width digitized using threshold value 12 is decreased from T1 to T0. In some cases, the signals can not be digitized because the pulse width is too narrow. Or in other words, reproduction of the signals is impossible.
Variation in the effect provided by inter-wave interference depends not only on how the data patterns are combined, but also on other factors, such as the reflectivity of an optical system and of an optical disk used for signal reading. There are two types of DVDs that are employed as data recording mediums: a DVD-ROM used specifically for read-only, and a rewritable DVD-RAM. A DVD-ROM has a smaller modulation ratio A (=Hmin/Hmax), which is defined as a ratio of the amplitude (Hmin) of the shortest data to the amplitude (Hmax) of the longest data, than does a CD-ROM; and a DVD-RAM has a greater change in light reflectivity than does a CD-ROM. As a result, both the DVD-ROM and the DVD-RAM tend to be affected by the inter-symbol interference more easily than is the CD-ROM.
A phase compensator for compensating for the effect of the inter-wave interference is disclosed in Japanese Unexamined Patent Publication No. Hei 6-333082. However, this device defines in advance a compensation value for a pulse width in accordance with a data bit pattern, and compensates for a pulse width of a digitally read signal by an amount that is the equivalent to the compensation value. Therefore, the conventional device merely compensates mechanically for the pulse width, in accordance with the data bit pattern, and does not determine a compensation-value by monitoring an actual read signal when compensating for the effect of the inter-wave interference, resulting in the precision of the signal digitization being low.